Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. patent application Ser. No. 12/421,057 (the '057 application) and Ser. No. 12/639,703 (the '703 application), the disclosures of which are hereby incorporated herein by reference in their entireties. The '057 and '703 applications disclose systems and methods for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and methods for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '057 and '703 applications are shown and described in U.S. patent application Ser. No. 12/879,595 (the '595 application), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '057 and '703 applications, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in the '595 application as well as in U.S. patent application Ser. No. 12/938,853 (the '853 application), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
In order to reduce overall pressure ranges of operation, CAES systems may utilize designs involving multiple interconnected cylinders. Unfortunately, this often results in trapped volumes of “dead space” at lower pressure than the gas. Such volumes may occur in the cylinders themselves and/or in the conduits interconnecting the cylinders, and may diminish the pressure of the gas in the system, thus reducing the amount of work recovered from or stored within the gas.
Air dead space tends to reduce the amount of work available from a quantity of high-pressure gas brought into communication with the dead space. This loss of potential energy may be termed a coupling loss. For example, if gas is to be introduced into a cylinder through a valve for the purpose of performing work by pushing against a piston within the cylinder, and a chamber or volume exists adjacent the piston that is filled with low-pressure gas at the time the valve is opened, the high-pressure gas entering the chamber is immediately reduced in pressure during free expansion and mixing with the low-pressure gas and, therefore, performs less mechanical work upon the piston. The low-pressure volume in such an example constitutes air dead space. Dead space may also appear within that portion of a valve mechanism that communicates with the cylinder interior, or within a tube or line connecting a valve to the cylinder interior. Energy losses due to pneumatically communicating dead spaces tend to be additive.
Moreover, in an expander-compressor system operated to expand or compress gas near-isothermally (at approximately constant temperature) within a cylinder, gas that escapes the cylinder to become dead space in a hydraulic subsystem may, as pressures change within the system, expand and compress adiabatically (at non-constant temperature), with associated energy losses due to heat transfer between the dead space and materials surrounding the dead space. Therefore, in various compressor-expander systems, including isothermal compressor-expander systems, preventing the formation of dead space will generally enable higher system efficiency.
Attempts to minimize such dead space frequently involve reducing the sizes and lengths of the conduits interconnecting the cylinders, but such efforts may not eliminate all dead space and, in any case, necessarily limit the overall geometry and placement of the individual system components.